Digital Image Correlation Analysis of Tibial Loading in Rotating Platform Total Knee Arthroplasty
Malinzak RM, Small SR, Rogge RD, Archer DB, Oja JW, Berend ME
Introduction
Mobile bearing total knee arthroplasty (TKA) tibial components can allow for high tibiofemoral
conformity while minimizing polyethylene contact stress and reducing bone-implant interface stress.
Prior studies have investigated the cortical strain variance between fixed and mobile bearing components
in a limited number of measurement regions. However, no current experimental data exists across the
entire proximal tibial cortex, and no comparisons have been made within the context of knee arthroplasty
revision surgery. The purpose of this study was to investigate the influence of bearing mobility on torque
and torsional strain across the entire cortical surface of the proximal tibia in the primary and revision
setting. Specifically of interest is the change in induced strain, the instance of femoral component
rotation and rotational malalignment.
Methods
In order to compare the mechanical response of the tibia following implantation of fixed and rotating
platform mobile-bearing TKA components, four experimental groups were included in this study: 1)
Fixed-bearing, posterior stabilized primary components (PFC Sigma, DePuy, Warsaw, IN); 2) Rotating
platform posterior stabilized primary components (PFC Sigma, DePuy, Warsaw, IN); 3) Fixed-bearing
posterior stabilized revision components with 115 mm press-fit distal stem (PFC TC3, DePuy, Warsaw,
IN; 4) Rotating platform posterior stabilized revision components with 75 mm press-fit distal stem (PFC
TC3, DePuy, Warsaw, IN). Six components in each experimental group were implanted into fourth
generation composite tibia specimens (Pacific Research Laboratories, Vashon, WA) using proximal
cementing and standard instrumentation.
Following implantation, tibias were prepared for surface strain quantification through the use of
complimentary strain gage and digital image correlation (DIC) methodologies. Two three-element
rectangular rosette strain gages were applied to the tibia in the anteromedial and posterolateral quadrants
for direct strain measurement. Digital image correlation techniques (Aramis 6.0, Gom, Inc.,
Braunschweig, Germany) were used to obtain full-field strain measurements 360 degrees around the
proximal tibial cortex. For DIC measurement, a black and white speckled paint was applied to the tibia
surface, which was then optically tracked with a set of two high definition cameras throughout the loading
cycle. To obtain full-field strain measurements around the entire tibia, tests were repeated in four
independent viewing angles. Deformation and strain measurements were calculated within the DIC
system software and then analyzed utilizing a custom merging algorithm (Matlab R2012a, Mathworks,
Natick, MA) to combine data from specimens and repeated trials within each experimental group.
Biomechanical testing was conducted on a biaxial electrodynamic materials testing machine (E10,000
A/T, Instron, Norwood, MA). Specimens were incorporated into the materials testing machine via a
custom fixture allowing free x-y translation of the potted base. Appropriate femoral components were
integrated into the upper testing grips to allow for repeatable load application through the femoral
component onto the polyethylene bearing surface. A silicon-based lubricant (DM-Fluid-350CS, ShinEtsu Chemical Co, Tokyo) was applied between all articulating surfaces to mimic in vivo frictional
characteristics. Testing was conducted in two phases: 1) Compressive loading followed by a 5 degree
internal rotation with femoral component in a full extension, and 2) Compressive loading followed by a
10 degree external rotation with the femoral component 90 degrees of flexion. In both instances the tibia
was loaded at a rate of 60 N/s to a peak load of 2.5 kN, while femoral component rotation was introduced
at a rate of 0.5 ˚/s. Five trials were repeated for each of the four DIC viewing angles in all 24 specimens.
Statistical analysis was performed utilizing paired t-tests to evaluate significant differences between
designs in torque response. Further examination was conducted to evaluate contribution of torsional
strain to the total overall strain response in each DIC measurement region. Statistical significance was
indicated at p ≤ 0.05.
Results
Torsional Response:
Average torsional moments during rotational testing in both 0 and 90 degrees of flexion are presented in
Table 1. In the primary setting, fixed bearing tibias generated 13.7 times the torsional moment of the
rotating platform primary design (p<0.01) when the extended femoral component was rotated 5 degrees
internally. Torsional moments in the fixed tibias were 11.2 times greater than those in the rotating
platform designs in the revision setting (p<0.01). In flexion, fixed bearing designs generated 4.4 times
greater torque in the primary (p<0.01) and 4.8 times greater torque in the revision setting (p<0.01) when
the femoral component was rotated 10 degrees externally.
Table 1: Torsional Response in 5˚ Internal Rotation (Extension) and 10˚ External Rotation (Flexion)
Fixed-Bearing
Rotating
(Nm)
Platform (Nm)
Extension
13.7 ± 3.2
1.0 ± 0.8
Primary
Flexion
16.0 ± 1.7
3.5 ± 0.7
Extension
15.7 ± 0.6
1.4 ± 0.6
Revision
Flexion
16.9 ± 3.0
3.5 ± 3.0
Strain Response:
Representative anterior and posterior DIC strain responses to compressive and torsional loading are
presented in Figure 1. Torsional strain response was seen to diminish substantially when rotating
platform devices were utilized, most notably in the posterior tibia. This diminished strain response to
torsional loading was consistent in both primary and revision tibial trays.
Figure 1: (A/B) Anterior/Posterior von Mises strain response to torsional loading in fixed-bearing primary
TKA in a single representative sample. (C/D) Anterior/Posterior von Mises strain response to torsional
loading in rotating platform primary TKA in a single representative sample. Measurement regions are
indicated as anterior “A” or posterior “P”
In order to numerically quantify DIC data, each field of view was divided into 5 measurement regions, in
order from most proximal to most distal, for von Mises strain averaging (Figure 1). Average cortical
strain for each measurement region was calculated by merging 8,000 – 30,000 individual strain data
points collected within the given region throughout five repeated trials of six specimens in each respective
experimental group. As a subset of all DIC strain analysis, the von Mises strain data for fixed and
rotating platform primary knee components, with 10˚ external femoral rotation is presented in Table 2. In
the primary fixed bearing components, average cortical strain in 6 of 10 measurement regions
significantly increased between 17% (p=.0004) and 56% (p=.0001) when the femoral component was
rotated 10˚ externally. Conversely, there was no statistically significant change in strain induced in the
primary rotating platform group with the introduction of external femoral rotation. In the fixed bearing
revision setting (not presented in Table 2), a significant increase from 18% (p=.0001) to 69% (p=.0001)
increase in strain was observed with 10˚ of external femoral component rotation in 6 of 10 measurement
regions. Similar to the primary components, there was no statistically significant change in strain due to
femoral rotation in any anterior or posterior measurement regions in the rotating platform design.
Table 2: Von Mises Strain Response to Loading in Fixed and Mobile Bearing Primary TKA Components
at 10˚ External Femoral Rotation
Posterior
Primary
Anterior
Fixed
Compression
Rotation
Mises Strain
(µm/m)
Mises Strain
(µm/m)
Compression +
Rotation
Mises Strain
(µm/m)
A1
362 ± 148
21 ± 166
A2
457 ± 191
A3
Compression
Rotating Platform
Compression +
Rotation
Rotation
Mises Strain
Mises Strain
(µm/m)
(µm/m)
p
Mises Strain
(µm/m)
384 ± 142
0.5591
457 ± 134
71 ± 111
528 ± 159
0.0665
158 ± 224
615 ± 332
0.0276
537 ± 223
12 ± 163
549 ± 211
0.8312
403 ± 124
32 ± 156
435 ± 183
0.4311
532 ± 154
28 ± 129
560 ± 154
0.4841
A4
544 ± 174
162 ± 127
706 ± 193
0.0012
735 ± 168
11 ± 128
745 ± 194
0.8317
A5
695 ± 234
236 ± 104
931 ± 231
0.0002
929 ± 241
39 ± 121
968 ± 235
0.5282
P1
642 ± 241
362 ± 236
1004 ± 299
0.0001
737 ± 340
46 ± 184
783 ± 348
0.6065
P2
651 ± 222
-49 ± 221
602 ± 279
0.4547
673 ± 270
8 ± 122
682 ± 261
0.8960
P3
1185 ± 213
65 ± 260
1250 ± 281
0.3168
1310 ± 263
10 ± 99
1320 ± 249
0.8803
P4
1145 ± 178
192 ± 133
1337 ± 213
0.0004
1186 ± 244
16 ± 115
1202 ± 216
0.7889
P5
1286 ± 176
251 ± 117
1537 ± 163
0.0001
1353 ± 271
34 ± 102
1387 ± 237
0.6069
p
Discussion
Femoral component rotation about the tibial tray occurs cyclically during the gait cycle and generates a
torsional moment at the tibial tray. Relative femoral component rotation and subsequent torsional
moments can also be generated in the event of suboptimal rotational positioning during implantation.
Both the primary and revision rotating platform designs exhibited vastly reduced torque response between
the articulating femoral and tibial components when under compressive loading and femoral rotation.
Strain response in the tibia was significantly altered in the majority of measurement regions when the
femoral component was rotated on the tibial tray in the fixed bearing designs. However, no significant
change in cortical strain loading was observed in anterior and posterior regions when the femoral
component was rotated in the rotating platform designs. The clinical application of this study may be
limited due to the use of composite, rather than cadaveric tibial specimens. Furthermore, no muscular or
ligamentous forces were replicated during loading. Nevertheless, this model is effective in the direct
comparison between strain and torsional response fixed and mobile-bearing designs.
Significance
In a comparison between fixed and rotating platform tibial component designs in both the primary and
revision setting, rotating platform tibial trays demonstrated significantly less transfer of rotational
moment between the femoral and tibial component than their fixed-bearing counterparts. As a result of
this diminished torsional load transfer, minimal increases in cortical strains were observed during femoral
component rotation in the rotating platform study group. The decrease in torque transfer between tibial
tray and implanted bone in mobile-bearing technology may act as a safeguard to reduce stress and
torsional fatigue at the bone-implant interface.